A Mechanistic Overview of the Cellular Pathology and Prion-Like Propagation of α-Synuclein in Parkinson's Disease: A Narrative Review

Abstract

Introduction: Prions are quaternary protein complexes made up of misfolded protein isoforms (PrPSc) that are able to aggregate and self-replicate in the absence of nucleic acids. They do this by incorporating and inducing the misfolding of normally-folded protein isoforms (PrPC) in a template-directed fashion, before fragmenting and continuing to propagate at increasingly greater concentrations in cells. Parkinson’s disease (PD) is a disease affecting the central nervous system (CNS), primarily characterised by a marked loss of dopaminergic neurons from the pars compacta of the substantia nigra in the midbrain (snPC). Lewy bodies (LBs) and Lewy neurites (LNs) are protein inclusions made primarily of the protein α-synuclein (α-Syn) and are implicated in defective neural signalling and neuronal cell death in PD. LBs/LNs have been hypothesised to cause symptoms of PD via propagation throughout the CNS in a prion-like mechanism. Methods: A narrative literature review was conducted to synthesise current and past research surrounding the prion-like propagation of α-Syn in PD, and models connecting cellular pathology to pathophysiology. Results were critically analysed and implications were determined. Results: α-Syn is a conformationally flexible protein normally involved in presynaptic regulation and dopamine homeostasis. In PD, α-Syn takes on a pathogenic, β-sheet rich conformation resulting from random cellular events or inoculation, that acts in a prion-like manner, inducing the misfolding of normal protein isoforms. Pathogenic α-Syn is propagated between neurons via exosomal secretion and leads to neurotoxicity by loss-of-function causing disruption of dopamine homeostasis, and proteasomal saturation and inhibition. Due to prion-like propagation, and differential susceptibility of neurons to pathogenic α-Syn-mediated neurotoxicity, models of PD progression and symptomatology have been suggested with differing degrees of success. Discussion: This narrative review aims to build on previous knowledge by clearly describing and evaluating the mechanisms of prion-like propagation and neurotoxicity of pathogenic α-Syn in PD, comparing them to traditional prion mechanisms observed for PrPSc, and models of PD which connect cellular pathology with pathophysiology. Conclusion: This review provides insight into the cellular mechanisms behind PD and can be used to propel research in this areas via identification of future areas of inquiry and pharmacological targets, among others.